Current vehicle emission standards will not mitigate climate change or improve air quality

The vehicle emissions testing programme was conducted by the UK Department of Transport in 2016 in response to emissions tampering exposed in the Volkswagen (VW) emissions scandal. The programme identified large emissions discrepancies between real-world and in-lab testing across a range of Euro 5 and Euro 6 diesel passenger vehicles. The large vehicle test fleet reflects the current challenges faced in controlling vehicle emissions. This paper presents the following findings: NOx emissions are altered due to exhaust gas recirculation mismanagement. A new Real-Life Emissions methodology is introduced to improve upon the current Real Driving Emissions standard. A large and concerning emissions divergence was discovered between the achieved NOx improvement and deterioration of CO2. The findings act as catalysts to improve vehicle emissions testing beyond standards established since the VW scandal, aiding in the development of better climate change mitigation strategies and bring tangible air quality improvements to the environment.

www.nature.com/scientificreports/ passenger vehicles. Based on these results, this paper assesses Exhaust Gas Recirculation (EGR) mismanagement during emissions testing, proposing a stricter Real Life Emissions (RLE) assessment method, and analysing the divergence between NO x and CO 2 emissions.

Exhaust gas recirculation mismanagement
The Vehicle Emissions Testing Programme delivered an independent assessment to identify software tampering strategies in passenger vehicles. A range of Euro 5 and 6 vehicles were chosen to encapsulate 75% of sales of the top 70 vehicles in the UK. This selection represents more than 50% of all diesel passenger cars models licensed and in use on UK roads 22 . Although no evidence of software tampering was found (apart from the VW case), major differences between in-lab and real-world emissions performances were exhibited. Figure 1 displays the various NO x emissions results of the Euro 5 (< 180 mg/km) and Euro 6 (< 80 mg/km) diesel vehicle fleets across (a) Euro  www.nature.com/scientificreports/ the three tests: NEDC hot, NEDC track and RDE. Results are expressed using the conformity factor (CF), defined as the ratio of the recorded emission value to its Euro limit. Although the vehicles conformed to their legislative limits in laboratory testing, emission levels are significantly higher in real world testing conditions. A large majority produced a CF value greater than 2 in either NEDC track or RDE testing and substantially exceeded this value. More specifically, the RDE tests revealed that around half of the Euro 6 fleet were only Euro 3 compliant on the road (< 500 mg/km, imposed in 2000 27 ), and most Euro 5 vehicles were not even Euro 1 compliant (< 970 mg/km, imposed in 1992 28 ). This raises concerns on the true real-world vehicle emission performances, their present compliance towards the Euro regulations even as of today in 2022, and the impact this may have on emissions modelling. Moreover, the NEDC track test follows the same NEDC speed-time profile but driven on a track to remove effects of in-lab conditions. One would expect a reasonably close agreement in emissions results between the two tests, but Fig. 1 displays major differences across the two tests. Several factors account for the emissions discrepancies. These include inaccurate in-lab aerodynamics replication (from the dynamometer coast down value setting), flexibility in vehicle testing configurations to obtain the best manufacturer emission results, presence of real-world cornering effects in track, and differences in ambient conditions (mainly temperature) 22 . However, even considering these factors, the differences in CF values are too large. More surprisingly, many vehicles performed similarly in both NEDC track (a 20 min drive cycle) and RDE (90 min of flexible on-road driving) tests. This led to the main hypothesis of analyzing the impact of ambient testing temperature in affecting emissions performances due to its effect on EGR activation within a vehicle.
Utilizing an EGR system reduces the production of NO x emissions. However, limiting its activation under low ambient temperatures is a common practice amongst manufacturers to prevent moisture condensation and deposit build-up within the EGR system 22 . This temperature dependent strategy is implemented to protect against engine degradation and maximize vehicle operating lifetime. To assess the validity of the EGR temperature strategy, Fig. 2 compares the emission rates in both NEDC track and RDE tests based on the differences in ambient testing temperature. White bars indicate that the NEDC track test temperature was higher than the RDE test temperature, and grey bars indicate otherwise. Given that the EGR is expected to deactivate under colder testing temperatures, one would expect that NEDC track emission points (orange) should be below RDE points (yellow) upon having a higher NEDC track testing temperature, and the reverse should also hold true. However, many vehicles do not exhibit this trend.
To identify a considerable change in emissions performance and hence EGR manipulation, this work has set an emission difference ( NO x ) of 100 mg/km and above to be of significance, as changes larger than this implies that the vehicle's emissions is operating under a different vehicle class (difference between Euro 5 and Euro 6), which is not acceptable. Defining temperature difference as T , vehicles that did not abide by the expected EGR-temperature strategy were categorized under 3 anomalies. The first anomaly displays the wrong temperature-emission correlation, whereby the NO x emission rate is lower in the colder test. The second anomaly represents a large change in ambient testing temperature ( T ≥ 1 • C ) yet changes in NO x emissions are small ( NO x ≤ 100 mg/km). Finally, the last anomaly is opposite to the second, whereby T ≤ 1 • C but NO x ≥ 100 mg/km. Table 1 lists the vehicles which demonstrates the respective anomalies. Vehicles under the first anomaly represent the most concerning group as the temperature variation does not produce the expected change in NO x emissions. This requires further investigation on a per vehicle basis. The second anomaly shows that the EGR deactivation temperature is not located within the T band despite undergoing large temperature changes. This indicates that the EGR deactivation point is prescribed at either a colder temperature, or the EGR system has already been activated at a hotter temperature and NO x emissions cannot be further decreased. Finally, the third anomaly shows that the EGR deactivation point is ideally located within the small T band, which explains the large change in NO x emissions.
The presence of these three anomalies further reinforces the wide variation in EGR strategies adopted across manufacturers and its impact in altering on-road NO x emissions. Moreover, a recent ruling by the EU Court of Justice has prohibited the disabling of such defeat devices for the purpose of minimizing engine ageing and degradation 29 . This represents a momentous first step towards including the consideration for EGR mismanagement in future emissions testing to enforce greater responsibility and liability from vehicle manufacturers. However, note that vehicles listed in Table 1 are not solely at fault, as Fig. 1 still shows that all vehicles were not compliant to the Euro limits under real world testing, regardless of considering EGR mismanagement. Other factors are also in play, and this requires further examination on a vehicle manufacturer case-by-case basis.

Real Life Emissions (RLE)
The RDE test successfully highlighted differences between in-lab and real-world emission values. However, the RDE drive cycle profile is legislated to contain approximately 60% of the time spent in urban driving and 40% in motorway driving 23 . This proportion split does not reflect the driving pattern of every passenger, given that there are numerous factors that influence daily travel patterns (such as geography, gender and age, trip purpose etc 30 ). While the RDE test has brought forward a much-needed level of transparency in emissions testing compared to in-lab methods, further improvements can be made. More specifically, there is a need to scale real-world emissions results from the fixed RDE drive cycle and apply it to different urban-motorway time proportions. Therefore, the Real Life Emissions (RLE) method was conceived in this paper to evaluate changes in emission levels with varying cycle compositions, and the RLE evaluates a new combined emission rate based on the selected time proportions. Derivation of the RLE is described in the Methods section. Figure 3 displays the RLE results of NO x and CO 2 emission rates for each tested vehicle, with values presented at full motorway, RDE (60% urban), and full urban driving compositions. For NO x emissions, it is evident that every vehicle contains a different emissions response with progression from full motorway to full urban driving. www.nature.com/scientificreports/ Three main groups can be classified based on their respective NO x emissions trends: Group 1-Increasing NO x emissions with greater urban driving, Group 2-Decreasing NO x emissions with greater urban driving, and Group 3-NO x emissions being relatively independent of drive cycle composition. Again, the same difference of NO x ≥ 100 mg/km was chosen to reflect significant change in emissions. Group 1 vehicles pose an immediate health hazard towards citizens close to the roadside due to higher NO x emissions in urban conditions where direct NO x exposure is of concern. In contrast, Group 2 vehicles contain higher motorway emission rates. Combined with longer travelling distances in motorway driving, such vehicles will produce greater cumulative NO x amounts. Finally, emission performances from Group 3 vehicles are representative of any driving scenario, as the NO x emission rates are approximately constant regardless of cycle composition.
For CO 2 emissions, Fig. 3 demonstrates a consistent increase in emission rate with greater urban driving across all vehicles. This is associated to the sustained power draw during start-stop urban driving alongside the shorter distance coverages, hence inflating CO 2 production per km. Although this negatively portrays vehicle driving under pure urban conditions, the lower motorway emission rates will be countered by the significantly longer travelling distances. Driving mileage still stands as the most important factor in determining the final cumulative CO 2 production from passenger vehicles.  www.nature.com/scientificreports/ Variances in NO x and CO 2 emission spectrums across different urban-motorway driving proportions accentuates the importance of integrating the RLE method, instead of solely relying on a single drive cycle composition result. Evaluation of air quality depends on accurate emissions modelling known as vehicle emission factors 31 . The RLE method helps to categorize vehicles based on their respective NO x emissions trends. These can then be applied on a local area basis to improve emissions factor modelling and bring better air quality policing. This method can also be similarly applied to improve predictions on cumulative emissions modelling across a nationwide scale 32,33 , which helps to enforce more impactful legislations targeted towards combating the finite global carbon budget.

Divergence in vehicle emissions
The current CO 2 target in the EU involves limiting vehicle emissions of new passenger cars to a fleet-wide average of 95 gCO 2 /km from 2020 34 . However, unlike the Euro standards, there is no quantitative limit on tailpipe CO 2 emissions for each individual vehicle. As such, the divergence factor was proposed to assess the increase in CO 2 emission rate from the in-lab NEDC hot test to the real-world RDE test. NO x divergence factors (ratio of RDE NO x to in-lab NEDC NO x emission rate) were also calculated to assess the NO x variance within each Euro fleet. Figure 4 shows the development of both CO 2 and NO x divergence factors from Euro 5 to Euro 6, reflecting the progression towards stricter emission limits from 2011 to 2016 10 and beyond, and Table 2 contains the list of labels for the Euro 5 and 6 vehicles in Fig. 4. Figure 1 shows that the average NO x conformity factor for the RDE test maintained at approximately 6.4 while progressing from Euro 5 to Euro 6, an improved (but still insufficient) reduction in real-world NO x emissions. However, Fig. 4 shows that the average NO x divergence has increased from Euro 5 at 5.81 to Euro 6 at 8.15, demonstrated by the wider vertical spread within the Euro 6 fleet. This is an undesirable outcome as the goal towards lowering NO x emissions hinders the ability to enforce emissions control uniformity across a fleetwide basis. On the CO 2 side, real world CO 2 emission rates from both vehicles fleets are similar (as observed in Fig. 3), but the progression from Euro 5 to Euro 6 also showed an increase in average divergence from 1.16 to 1.26, again indicating the larger unwanted CO 2 emissions variance.
A study conducted by the International Council on Clean Transportation (ICCT) covering an enormous dataset of 1.3 million vehicles across eight European countries, demonstrated an average increase in CO 2 divergence Table 1. Identification of vehicles exhibiting anomalies in the EGR temperature strategy. Cells with + represent positive display of the associated anomaly.

Description of anomalies
Wrong temperature-emission correlation www.nature.com/scientificreports/   (a,b) NO x divergence factor (a), CO 2 divergence factor (b).The divergence factor is defined as the ratio of the RDE emission rate to the in-lab NEDC emission rate. Circular scatter points represent the divergence factor corresponding to the in-lab emission rate, and each point is denoted by the label of each tested vehicle as listed in Table 2. Advancing from the blue to orange diamond scatter point represents the development of the average divergence ratio moving from the Euro 5 to Euro 6 fleet. Note that the true points for 2 J in (a) and 1 M in (b) are located far outside of the axis limits. They are located at (36.8 mg/km, 28.1) and (267.7 g/km, 0.580) respectively. www.nature.com/scientificreports/ variances have worsened. This calls for the need to emphasize the importance of future simultaneous control of both emission types.

Discussion
The Vehicle Emissions Testing Programme demonstrated the inadequacy of the laboratory NEDC test in reflecting real-world emissions. Results from the NEDC track and RDE tests reveal real-world NO x production from both Euro 5 and 6 vehicles are only Euro 3 compliant at best. The latest real-world regulatory standard RDE4 requires all new vehicles (1st January 2021 onwards) to abide towards a conformity factor of 1.43 or lower, with the 0.43 margin accounting for uncertainties in emissions measurements 26,39 . However, the tested vehicles clearly exceed this threshold, and future testing policies need to be sufficiently demanding to ensure real-world emissions are drastically improved upon moving towards Euro 7. Furthermore, certain vehicles did not exhibit the expected changes in NO x emissions despite differences in ambient testing temperatures, and 3 main anomalies related towards the EGR-temperature activation strategy were highlighted to account for the observation. It was determined that EGR activation mismanagement stands as a prime driver in producing such varied NO x emissions results, and EGR manipulation necessitates greater attention and consideration in future emissions testing. The RLE method strengthens the effectiveness of the RDE test through the creation of emission spectrums with varying urban-motorway driving proportions. Its application further reveals differences in NO x emissions performances, with vehicles performing better or worse in full urban conditions, or having stable emission rates that are independent of drive cycle composition. Moreover, increases in CO 2 emission rates with greater urban driving was observed across all vehicles. The proposed RLE method could stand as an integral component for emissions modelling given its improved testing transparency and strong applicability towards any emissions standard. As implementation of emissions standards (such as the Clean Air Zones) relies on accurate modelling of the vehicle emission factors 31 , the RLE method could greatly reduce the modelling uncertainties. By accounting for variations in individual driving patterns and habits 30 across regions, the RLE method can be integrated to implement effective emissions policing, bringing in tangible air quality improvements across environments scaling locally to nationwide.
Lastly, although the progression from Euro 5 to Euro 6 successfully reduced the average NO x emission production, the divergence of both NO x and CO 2 emission rates has worsened. More importantly, the control of CO 2 emissions has been neglected as real-world emission rates from both vehicle fleets are similar. As such, elevating awareness for the simultaneous improvement in both CO 2 and NO x emissions is imperative in hopes of prohibiting the same trend from occurring while advancing to the final Euro 7 standard in 2025. Additionally, this is pertinent towards low-or middle-income nations outside of the EU which are either on or advancing towards the Euro 5 or 6 emissions standards, and countries who utilizes second-hand vehicles from the UK. Prevention of this divergence must be undertaken.
In conclusion, the Vehicle Emissions Testing Programme has served as a foundation in displaying the inadequacy of the laboratory testing in portraying real-world emission levels. While the in-lab NEDC standard has been replaced by the Worldwide Harmonised Light Vehicles Test Procedure 25 alongside RDE testing, further improvements in emissions testing need to be delivered in the form of EGR activation control, the RLE method, www.nature.com/scientificreports/ and emissions divergence assessment. Euro 5 and 6 vehicles are still prevalent in the ongoing transition towards transport decarbonization. As such, these methods stand as powerful, next-generation metrics to further improve vehicle emissions testing and reduce uncertainties in future emissions modelling for better climate mitigation policies. Integration of these in-depth emissions analysis will aid in the proposal of more impactful on-road emissions legislations and smarter traffic management, thus helping to avoid additional emissions-related premature deaths and push towards achieving the long-term goal of net-zero carbon by 2050.

Methods
Data for the Vehicle Emissions Testing Programme is freely available from the gov.uk website 22,23 . Testing data are provided for the three tests discussed in this paper: NEDC hot, NEDC track and RDE, specifically dealing with data for CO 2 emissions, NO x emissions and ambient testing temperature. The RLE methodology assess variations in the combined emission rate value upon changing the composition of the RDE drive cycle. Decomposition of the urban and motorway driving data was dependent on the respective drive cycle profiles. Urban driving consists of low driving speeds mainly under 48 km/h with multiple start-stop occurrences, whilst motorway driving consists of sustained periods of high-speed driving above 48 km/h. Upon separation of the urban and motorway portions, the average urban emission k d u and average motorway NO x emission k d m per unit distance (g/km for CO 2 or mg/km for NO x ) can be determined by: where k represents either NO x or CO 2 , k t u and k t m represents the urban and motorway emission per unit time respectively (g/s for CO 2 and mg/s for NO x ), d u and d m represents the urban and motorway distances respectively, and t u and t RDE represents the urban and the final RDE cycle time duration respectively. The motorway time duration t m is equal to t RDE − t u .
In an arbitrary driving cycle composition, the combined emission rate per unit distance k d c can be expressed by the following equation: where d RDE is the total RDE driving distance. Defining the urban and motorway distance proportions as D u and D m : However, using time proportions spent in urban and motorway driving ( T u and T m ) serves as a better representation towards characterizing the driving habits of individual passengers: Therefore, it is more appropriate to express the two distance proportions as functions of their individual time proportions, where D u/m = f t u/m . Derivation of the respective functions are as follow: where V u and V m represents the average urban and motorway speed of the drive cycle respectively. Dividing Eq. (9) by t RDE :